Bug fix: preventing a Computation result to be recomputed everytime it's queried because of unmatching hash.

Added a test showing the situation where it can happen: if the computation result is registered artificially and the hash of the given object does not match the real computation result, then it would never update the internal computation hash and would force the computation every time.
Adding warning message when the stored hash differs from the computed Object's hash.
Merge pull request #121 from ldmoser/recomputationBug

Bug fix: preventing a Computation result to be recomputed everytime